The present invention relates to a display device matrix, and more particularly, to a Liquid Crystal Display (LCD) panel matrix wherein the electrical connection between respective scanning line electrodes and bonding pads can be repaired to increase matrix yield and improve display device image quality.
An LCD panel displays data using a plurality of display cells optically actuated or switched by a matrix of signal lines. That is, specifically identified portions of the liquid crystal, i.e., pixels, are defined and operated by two sets of signal lines, data line electrodes and scanning line electrodes, orthogonally disposed in a matrix pattern.
FIG. 1 is a schematic diagram of a conventional LCD panel including a substrate 101, a pixel portion 102, signal lines 103 and bonding pads 104. Signal lines 103 are disposed in a matrix pattern, and switching elements such as thin film transistors or diodes are disposed at the respective intersecting points. Signal lines 103 are generally composed of metal, such as aluminum, having a very low resistivity.
Low resistivity signal lines inhibits deterioration of liquid crystal image quality due to signal delay. Scanning line electrodes exhibit a distinct RC time delay due to the gate and line capacitance. Signal delay becomes more pronounced as an LCD panel increases in size. Thus, aluminum signal lines having low resistivity are generally mandatory for large size LCD panels in order to avoid excessive signal delay and the resulting deterioration in image quality.
Unfortunately, aluminum has a low pressure-resistance property which generally means that a substantial portion an aluminum gate must be used as a bonding pad. Thus, as shown in FIG. 2, respective bonding pads 104 composed of a metal other than aluminum, such as, for example, chromium, having a high pressure-resistance property are coupled to respective signal lines 103.
A detailed description of the bonding pad 104 to signal line 103 connection will be made with reference to FIG. 3 which illustrates a cross-sectional view taken along line AA' of the structure shown in FIG. 2. In FIG. 3, signal line 103 is formed on a transparent glass substrate 101. An oxide film 105 is formed by anode oxidation over signal line 103, except at an end portion of signal line 103. Bonding pad 104 is formed over and electrically coupled to the end portion of signal line 103. An insulating layer 106 is then formed over bonding pad 104 and oxide film 105. In this arrangement, oxide film 105 prevents the formation of hillocks and pin holes in signal line 103. Insulating layer 106 is typically composed of a material like silicon nitride having a multi-layer structure such that it provides improved etching-prevention and excellent insulating characteristics.
Unfortunately, a variety of mishaps may result in a connection failure between bonding pad 104 and signal line 103. A connection failure may result from a step coverage defect, from a heat stress difference between bonding pad 104 and signal line 103 during a subsequent processing step, or from corrosion of signal line 103 by etchant infiltrating through oxide film 105. Connection failures reduce the pixel yield in the display device matrix and generally reduce image quality.
U.S. Pat. No. 4,807,973 proposes an LCD panel having a repair ring as a means by which the above-described problem can be remedied. However, this remedy increases the overall size of the LCD panel and signals transferred via a repair ring are unduly delayed. Furthermore, the number of connection defects which can be repaired by the repair ring is small. Thus, a satisfactory repair mechanism for connection failures in a large LCD panel matrix does not yet exist.